Posts Tagged scp

ipv6 urls

URLs are written like this: protocol://host-or-address:port/path-or-function

What happens with ipv6 is that addresses contains colons (“:”) , so how do you specify the port number in your web browser ? The same happens when you do an scp: you usually do scp user@host:path/to/file /local/path, how can you differenciate the host part and the path which are also seperated with a colon ?

The answer is: USE BRACKETS !
an ipv6 url can be written like this:

Also, a scp command with ipv6 addresses can be like this:

  • scp user@[fe80::abcd:abcd:abcd:abcd]:/etc/resolv.conf /tmp

I hope it’s usefull !

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scp with little files, use ssh and tar

scp is slow at copying a lot of little files. It’s probably because, it creates a ssh connection and then runs in a loop that open the file locally , reads it, sends it, creates the file remotely and restarts that loop. I think that the cost of opening files and creating files is high and tar is a lot more efficient at it (if you know exactly why, please post a comment). That’s why I often use ssh with a combination of tar to copy a directory with a lot of little files (let’s say a website with a tone of 10kB php files).

To achieve this, where I typically use:
scp -r a-directory/ user@host:
I would use:
tar cfz - a-directory | ssh user@host "tar zxvf -"

First, I create a tar file that output to stdout (tar cf -, the “-” is to output to stdout) then I pipe the output to an ssh connection that execute a command that read stdin and pass it to tar (tar xvf -, where “-” is stdin).

Surely, you can remove the “z” in the input and output tar , so the content won’t be compressed with gzip, or replace it with “j” to use bzip (with gnu tar).

that would be:

tar cf - a-directory | ssh user@host "tar xvf -" or
tar cfj - a-directory | ssh user@host "tar jxvf -"

Also, tar knows how to use ssh:
tar cvfz user@host:file.tar a-directory but this method have the drawback of creating a tar file remotely, not copying the directory, you still need to uncompress the tar on the other side.

Now some numbers, this is not a benchmark, this is just an indication, I had the idea of writing this article while copying a directory I needed to copy. Numbers may vary with size of files, number of files, local cpu, remote cpu, network bandwidth and probably other parameters:

# du -sh a-directory
109M a-directory
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recursive scp and symlinks

To copy recursively with scp, you use scp -r.
The thing is that if you have symlinks in your directory the content of the directory being pointed to by the symlinks will be copied, this is not necessarily what you want (the symlink will be followed instead of being preserved).
Scp have no option to specify that you don’t want to follow symlinks.
If you want to preserver symlinks, you should use rsync:
rsync -avz -e ssh /src/dir

Be carefull, if you preserve symlinks, the newly created symlinks on the remote server can point to a non existant path.
It’s strange that scp seem to have no option to not follow symlinks. If you’re aware of one, please drop me a comment !

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